The use of polyethylene to produce films suitable for a variety of commercial products is generally well known. Different methods may be employed to form such polyethylene films. Such methods include, but are not limited to, extrusion-blown film process, and extrusion-cast film process. Each process has its own advantages and disadvantages. The main difference between these two processes is the manner of cooling an extruded sheet of molten polymer.
The extrusion-blown film process is a well know process for preparation of plastic films. The extrusion-blown film process employs an extruder which heats, melts, and conveys the molten polymer and forces it through an annular die. The molten polymer is drawn from the die and formed into a tube shape morphology and eventually passed through a pair of draw or nip rollers. Internal compressed air is then introduced from the mandrel causing the tube to increase in diameter thereby forming a bubble of a desired size. External air is also introduced around the bubble circumference to cool the melt as it exits the die. Film width is typically varied by introducing more or less internal air into the bubble thus increasing or decreasing the bubble size. Film thickness is typically controlled by increasing or decreasing the speed of the draw roll or nip roll to control the draw-down rate. The bubble is then collapsed into two doubled layers of film immediately after passing through the draw or nip rolls. The cooled film can then be processed further by cutting or sealing to produce a variety of consumer products.
Furthermore, film properties may be influenced by both the molecular structure of the polymer and/or by the processing conditions, e.g. blow up ratio, i.e. the ratio of the diameter of the blown bubble to the diameter of the annular die.
Improved film properties such as improved optical properties and improved mechanical properties are clearly desirable for a variety of applications. Different techniques have been proposed to achieve such improved properties.
U.S. Pat. No. 4,346,834 discloses blended polyolefin resins for making seamless-wall handled strap bags from thin tubular film consisting essentially of a homogeneous blend of HDPE, LLDPE, and ordinary branched LDPE.
U.S. Pat. No. 4,377,720 discloses a process for preparing linear alpha olefins and waxes having a total product Mn in the range 200 to 700 comprising at least 90 mole percent linear alpha olefins which includes polymerizing an ethylene containing gas in the presence of the reaction product of a zirconium halide compound with an aluminum cocatalyst selected from the group consisting of dialkyl aluminum alkoxides or aryloxides and dialkyl aluminum disubstituted amides in the presence of a diluent at a temperature of about 50° to 200° C. and an ethylene pressure above about 3.5 MPa, wherein the Mn of the reaction product is controlled by the molar ratio of the aluminum cocatalyst/Zr halide, and the molar ratio being less than about 1.
U.S. Pat. No. 4,533,578 discloses a 3-ply trash bag prepared by a sandwich foam, blown film co-extrusion process. The skin layers are composed essentially of high performance polyolefins such as linear low density polyethylene. The foamed middle layer imparts to the bag improved structural integrity and stiffness, an independent tear mechanism for the skin layers, and improved TD tear strength.
U.S. Pat. No. 4,579,912 discloses that linear low density copolymers (LLDPE) are blended with up to about 10 weight percent of an aromatic polymer, e.g., polystyrene or poly(para-methylstyrene), to give films having improved MD tear strength or equivalent MD tear strength and higher stiffness compared to the LLDPE alone.
U.S. Pat. No. 4,614,764 discloses that the properties of linear ethylene polymers, especially linear low density polyethylene (LLDPE), are improved by adding a chemically modified ethylene polymer to the base resin. The modification is brought about by the use of a free radical generator, preferably, an organic peroxy compound such as benzoyl peroxide, optionally with an unsaturated silane such as vinyl trimethoxy silane.
U.S. Pat. No. 4,657,811 discloses a three-ply plastic film prepared by a sandwich foam, blown film co-extrusion process. The outer skin layers are composed of high performance polyolefins, such as linear low density polyethylene. The foamed middle layer imparts to the film improved structural integrity and stiffness, an independent tear mechanism for the skin layers, and improved TD tear strength.
U.S. Pat. No. 4,716,201 discloses that linear low density copolymers (LLDPE) are blended with up to about 10 weight percent of an aromatic polymer, e.g., polystyrene or poly(para-methylstyrene), to give films having improved MD tear strength or equivalent MD tear strength and higher stiffness compared to the LLDPE alone.
U.S. Pat. No. 5,000,992 discloses a plastic container closure, such as a bottle cap liner or tamper evident seal, which is formed from a co-extruded multilayer foamed film. The co-extruded multilayer foamed film has at least one solid film layer of a first polyolefin blend containing linear low density polyethylene, low density polyethylene, and optionally high density polyethylene, and at least one foamed layer of a second polyolefin blend containing linear low density polyethylene, low density polyethylene, and optionally ethylene vinyl acetate.
U.S. Pat. No. 5,210,167 discloses a film of linear low density copolymer of ethylene having an MZ/MW ratio of greater than 3.5.
U.S. Pat. No. 5,258,463 discloses a film formed from a blend composition comprising a linear low density copolymer of ethylene and a minor amount of an olefin containing 4 to 10 carbon atoms.
U.S. Pat. No. 5,569,693 discloses a stretch/cling film suitable for pallet wrap, produced by a blown film process. This film is produced by blending 40 to 80% by weight of linear low density polyethylene polymerized with an alpha-olefin comonomer having a melt index range of 0.8 to 1.2 and a density range of 0.916 to 0.920; with 60 to 20% by weight of a flexible semi-crystalline polyethylene with a melt index of 0.8 to 2.0 and a density of 0.86 to 0.91; with 3 to 9% by weight of liquid polyolefin and 0.1 to 4.0% by weight of a surfactant.
U.S. Pat. No. 6,204,335 discloses a film produced from a composition comprising about 80 to about 99 weight percent, based on the composition, of a linear ultra low density polyethylene and about 1 to about 20 weight percent, based on the composition, of a propylene polymer comprising 0 to about 40 weight percent olefin comonomer units having 2 to 10 carbon atoms and from about 60 to about 100 weight percent propylene, based on the propylene polymer, the propylene polymer having a Brookfield Thermosel viscosity of about 1 to about 30,000 mPa·s at 190° C.
U.S. Pat. No. 6,340,532 discloses shrink films manufactured from pseudohomogeneous linear low density polyethylene resin. The pseudohomogeneous resins are prepared with a Ziegler Natta catalyst, preferably in medium pressure solution polymerization process.
U.S. Pat. No. 6,500,901 discloses a film produced from a composition comprising about 80 to about 99 weight percent, based on the weight of the composition, of a linear ultra low density polyethylene and about 1 to about 20 weight percent, based on the weight of the composition, of a propylene polymer comprising 0 to about 40 weight percent olefin comonomer units having 2 to 10 carbon atoms and from about 60 to about 100 weight percent propylene, based on the propylene polymer.
U.S. Pat. No. 6,696,166 discloses a plastic film prepared from a pseudohomogeneous linear low density polyethylene resin under specific extrusion conditions using a specific annular extrusion die.
U.S. Pat. No. 6,767,599 discloses a process for making pouches filled with a flowable material, using a vertical form, fill and seal apparatus, wherein the pouch is made from a blown film formed from a blend comprising a linear polymer of ethylene with at least one C4-C10 alpha-olefin manufactured in a single-site catalyst polymerization process, and at least one of the following (a) a linear polymer of ethylene with at least one C4-C10 alpha-olefin made by a multi-site catalyst polymerization process; (b) a high pressure low density polyethylene; and (c) additives selected from the group consisting of stabilizers, antiblock additives and extrusion aids.
Despite the research efforts in developing polyethylene films with improved properties such as improved mechanical properties and/or improved optical properties, there is still a need for new polyethylene films having both improved mechanical properties and improved optical properties, and method of making the same.